Crayfish under duress show signs of anxiety, scientists report in the June 13 Science. The results provide clear evidence of the complex behavior in invertebrates and hint at anxiety’s ancient origins.

To look for signs of fretting, French researchers watched the lobsterlike crustaceans explore a cross-shaped tank that had two dark arms and two lit ones. Crayfish preferred the safety afforded by the dark, but they also occasionally explored the illuminated arms. However, compared with normal crayfish, animals subjected to a series of stressful shocks before entering the tank spent less time exploring lit areas and more time huddled in the dark.

Anxious behavior lessened after researchers treated the crayfish with chlordiazepoxide, an antianxiety drug for people. The scientists also found that an injection of serotonin, a mood-regulating chemical messenger, induced anxious behavior in unstressed crayfish.

Crayfish join rats, mice, zebrafish and other animals that possess the ability to become anxious. The growing tally suggests that anxiety evolved long ago to help animals survive.